1. Field of the Invention
The present invention relates to a hydraulic control system for a working machine such as a hydraulic excavator.
2. Description of the Related Art
As hydraulic pump control systems for working machines such as hydraulic excavators, there are known a positive control system and a negative control system. In both control systems, a pump controller is composed of a pump regulator for controlling a discharge amount of a hydraulic pump, a regulator control valve of electromagnetic proportional type for controlling an operation of the pump regulator, and a controller for controlling the regulator control valve.
In such electronic control systems, however, once there occurs failure of the electromagnetic proportional valve itself or of the control system such as breaking of wire in a signal system which is for sending a control signal from the controller to the regulator control valve, a pump controlling oil pressure provided from the regulator control valve vanishes (or lowers), with the result that the pump discharge amount becomes minimum and a normal actuator operation cannot be performed.
Consequently, in the working machine concerned, it becomes impossible to actually carry out a work at a working site. For this reason, it has actually been difficult to adopt the electronic control systems for the hydraulic pump.